Developments in combustion engine technology have shown that compression ignition engines, frequently referred to as diesel-cycle engines, can be fuelled by gaseous fuels instead of diesel without sacrifices in performance or efficiency. Examples of such fuels include natural gas, methane, propane, ethane, gaseous combustible hydrocarbon derivatives, hydrogen, and blends thereof. Substituting diesel with such gaseous fuels generally results in cost, availability and emissions benefits over diesel.
However, one challenge in using gaseous fuels for such applications is that the energy density of gaseous fuels is not as high as conventional liquid fuels. This is an important consideration, for example, with gaseous fuel systems employed for vehicular applications, because fuel storage space on-board a vehicle is limited. For gaseous-fuelled vehicles to be attractive versus conventional diesel-fuelled vehicles, the on-board fuel storage system should not diminish the vehicle's transport capacity or range.
To increase the energy density of a gaseous fuel, it can be stored at high pressure. To contain a gas at high pressure, a pressure vessel rated for a specified maximum holding pressure must be used. For gaseous fuels, compared to a compressed gas, higher energy densities can be achieved at lower storage pressures with a liquefied gas. As a result, the fuel tank does not need to be rated for as high a pressure, which can reduce the weight of the fuel tank.
Accordingly, a preferred method of increasing the energy density of a gaseous fuel is to store it in a liquefied state at cryogenic temperatures. A liquefied gas stored at a cryogenic temperature is referred to herein as a cryogenic fluid and a gaseous fuel stored in a liquefied state at a cryogenic temperature is referred to herein generally as a “cryogenic fuel”.
For the purposes of this application, cryogenic fuels include liquefied gaseous fuels that boil at temperatures at or below −100° C. under atmospheric pressures. An example of such a fuel is liquefied natural gas, commonly known as “LNG”.
In the present disclosure LNG is referred to as a preferred example of a cryogenic fuel because of the vast proven reserves of natural gas in many of the potential markets around the world, the affordability of natural gas, and the already existing infrastructure for natural gas, which is continuing to expand in breadth and capacity.
However, people skilled in the technology would understand that the presently disclosed storage container can be employed to hold other cold or cryogenic fuels or liquefied gases generally. By way of example, the disclosed storage container could be employed to accommodate other hydrocarbons such as methane, ethane, propane and hydrocarbon derivatives or non-organic fuels such as hydrogen. Furthermore, the container that is the subject of this disclosure can also be used to hold other liquefied gases at cryogenic temperatures, such as helium, nitrogen and oxygen.
However, one of the challenges of storing liquefied gas at cryogenic temperatures is reducing heat transfer into the cryogen space. As the temperature of the liquid increases, the vapour pressure rises inside the storage container. Cryogenic storage containers are normally equipped with a pressure relief venting system to prevent over-pressurization of the storage container. Excessive heat transfer into a cryogen space can result in fuel wastage through venting. When the liquefied gas is a fuel, it is also undesirable to routinely release fuel into the environment. With natural gas used as an example, methane, a major component of natural gas, is a greenhouse gas.
Cryogenic storage containers commonly use a double-walled construction with an inner vessel, which holds the liquefied gas, suspended inside an outer vessel. A vacuum applied to the space between the inner vessel and the outer vessel minimizes or reduces conductive and convective heat transfer. It is also known to wrap sheets of a super-insulating material around the inner vessel to minimize or reduce radiant and convective heat transfer. However, the structural supports for the inner vessel, as well as piping between the inner vessel and outside environment, provide heat conduction paths and the transfer of heat energy to the liquefied gas in the cryogen space from the outside environment is commonly known as “heat leak”.
As long as there are structural supports for the inner vessel and there are pipes or conduits that penetrate through the insulated space, some heat leak will occur. “Holding time” is defined herein as the time span that a cryogen can be held inside the storage container before heat leak into the cryogen space causes the vapour pressure to rise to a level at which the pressure relief valve opens. Accordingly, holding times can be extended without the need to vent excess vapour pressure if heat leak can be reduced.
For containers with an elongated horizontal axis, much of the conventional technology for supporting the inner vessel suspended inside the outer vessel was developed for relatively large storage tanks such as ones that are used for bulk transport of a cryogen onboard a ship, train, or trailer, or for a stationary storage vessel used at a dispensing station. The smallest of these types of containers typically has a volumetric capacity that is greater than 3,500 liters, and some of the larger bulk storage containers can have a volumetric capacity that is greater than 380,000 liters. With such large storage tanks, a higher amount of heat leak can be accepted because the volume of the container and the mass of the cryogen stored therein is orders of magnitude greater than that which is associated with a smaller vehicular fuel tank, which typically has a volumetric capacity of 450 liters or less. That is, a larger mass of cryogen stored in a larger container can absorb a much greater amount of heat before the vapour pressure rises to a predetermined relief pressure, compared to a smaller mass of cryogen stored in a smaller container. If conventional technology for supporting the inner vessel of a large storage tank is applied to a smaller container, the heat leak into the cryogen space through the support structure could result in shorter holding times, compared to the holding times that can be achieved when the same type of support structures are used for a larger tank, even when the support structure for the smaller vessel is appropriated scaled in size. Such shorter holding times may not be acceptable depending upon the application for the smaller vessel. For smaller containers such as vehicular fuel tanks longer holding times are advantageous, and it is desirable to have support structures that can provide reduced heat leak, compared to conventional inner vessel support structures designed for much larger storage containers.
Compared to conventional liquid fuels like diesel oil and gasoline, the use of fuels stored on-board a vehicle at cryogenic temperatures is relatively new. Now that relatively small containers for cryogenic fluids are being considered for this mobile application, conventional support structures originally designed for larger bulk storage containers have been adapted. With larger containers the structural support that can be provided by piping between the inner and outer vessel is ignored since it is nominal, as is the heat leak that can occur through such pipes, given the size of such containers and the mass of cryogen held therein. For smaller containers, the thermal conductivity through the pipes that penetrate through the insulated space can be considerable, especially since pipes are typically made from metallic materials that have significant thermal conductivity. Accordingly, to improve the holding time, improvements are needed to reduce the heat leak into smaller containers that can be used to store a fuel on-board a vehicle at cryogenic temperatures.
There are known containers for holding a cryogenic fluid that are smaller than the bulk transport containers described above, but these containers generally have a vertical longitudinal axis, with the fluid connections into the inner cryogen space associated with the top of the container. Because the vapour space occupies the upper region of these vertically oriented containers, the heat leak through the conduits at the top of the container is reduced because all or at least a part of the conduit is not in direct contact with the cryogenic liquid. The support structures for the inner vessel of a double-walled vacuum insulated container are dependent in part upon the configuration of the container and whether the container is designed for stationary or mobile use. Accordingly, although there are known containers with a volumetric capacity that is generally the same as that required for storing a cryogenic fuel on-board a vehicle, these known containers are not suitable for this relatively new application. For example, containers with a vertical longitudinal axis are not suitable for mounting on-board a vehicle at the location where fuel tanks are normally mounted, and containers intended for stationary use are not suitable for mobile use because of the dynamic loads that must be carried by the support structure for the inner vessel.